1. Field of the Invention
The present invention relates to a Cognitive Radio (CR) system. More particularly, the present invention relates to a method and apparatus for switching operational modes in a CR system.
2. Description of the Related Art
CR systems scan a spectrum of frequencies in a licensed system, determine which frequencies or channels are not in use and operate using the unused frequencies. When the CR systems have multiple unused channels available for operation, in order to minimize mutual interference with neighboring systems, the CR system tends to select a channel that is different from those used by the neighboring systems. This is referred to as a ‘spectrum etiquette mechanism’.
The CR systems exchange basic information such as an operational channel, the availability of channels, spectrum sensing scheduling, etc. through communication between Base Stations (BSs). By acquiring operational channel information on neighboring systems, the CR system selects an operational channel from channels of the neighboring systems. In order to minimize a mutual interference, the CR system has to keep a mutual distance in a frequency domain.
The spectrum etiquette mechanism operates only when there are a plurality of selectable available channels. When there are an insufficient number of available channels, for example, when sensed channels of a licensed band are all occupied by the neighbor systems, a different mechanism is applied to address this scenario.
According to the Institute of Electrical and Electronics Engineers (IEEE)802.22 standard, a channel renting/offering or channel contention mechanism is performed if the spectrum etiquette mechanism fails to provide sufficient spectrum resources. In other words, when the CR system fails to acquire an operational channel by the spectrum etiquette mechanism, the CR system rents a channel already occupied by another neighbor system, or occupies an operational channel through contention. However, the channel renting/offering and channel contention mechanisms are not efficient under the following environment. First, when a BS1 rents a channel from a BS2, this means that, after a negotiated period, the BS1 returns the rented channel to the BS2. This does not coincide with a rule of a license-exempt environment in which a CR system rents and uses a licensed frequency band with another system with the same right/priority. A cell does not possess a specific channel. Therefore, when the BS2 has a plurality of available channels, if the BS1 contends a channel (N) against the BS2 but fails in the contention, the BS2 may keep operating in the channel (N).
Thus, there is a need for a method and apparatus for switching between a channel-based sharing mode and a frame-based sharing mode in a CR system.